1. Field of the Invention
The present invention relates to a filter device used mainly in a microwave or millimeter wave band and, more particularly, to a distributed constant filter in which various wiring patterns are formed as circuit devices, a method of manufacturing the distributed constant filter, and a distributed constant filter circuit module.
2. Description of the Related Art
In a cellular telephone system such as a portable telephone or car telephone, or a communication system such as a wireless LAN (Local Area Network) using high frequency radio waves in the microwave band or millimeter wave band as carriers, a filter device such as a low pass filter (LPF), high pass filter (HPF), or band pass filter (BPF) is usually designed not as a lumped parameter line or a concentrated constant circuit but as a distributed constant circuit (or a distributed parameter circuit). The lumped parameter line is a circuit in which the physical size of a device as a component of the circuit is sufficiently smaller than the wavelength of an electric signal and which uses chips such as an inductance L and a capacitor C as circuit devices. The distributed constant circuit is constructed by using microstrip lines which will be described hereinlater and uses various wiring patterns each having the length that is about the same as the wavelength of an electric signal as circuit devices.
FIG. 15 shows a plan view of a BPF having microstrip line patterns formed in one plane on a dielectric substrate. The BPF shown in the diagram has a structure such that a plurality of narrow microstrip lines 102(1) to 102(5) made of a conductor such as copper are disposed in parallel so as to be apart from each other at predetermined intervals on a dielectric substrate 101 made of a material such as ceramic. The neighboring microstrip lines are disposed so as to be staggered each other in the longitudinal direction in such a manner that a part of a length, which is about the quarter of a pass wavelength xcex, of one of the neighboring microstrip lines overlaps with that of the neighboring microstrip line. The microstrip lines 102(1) to 102(5) can be simultaneously formed in a process of forming a wiring pattern on a wiring board performed by printing or lithography.
In the BPF of the configuration of using such microstrip lines, for example, an RF signal RF1 supplied from an end of the microstrip line 102(1) passes through the microstrip lines 102(1) to 102(4), during which high frequency components except for a component of the wavelength xcex in the RF signal RF1 are eliminated. Only an RF signal RF2 of the wavelength of xcex is outputted from an end of the microstrip line 102(5). When it is assumed that the wavelength of a radio wave in a space is xcex0 and the effective dielectric constant of the substrate is xcex5w, the pass wavelength xcex is given by the following equation (1). By optimizing the pattern of the microstrip lines 102(1) to 102(4), therefore, RF signals in a desired frequency band can be selectively allowed to pass.
xcex=xcex0/(xcex5w)xc2xdxe2x80x83xe2x80x83(1) 
In recent years, also in the uses of high frequencies, a demand of reducing the size of a device and a substrate has been becoming stronger. In the BPF of the configuration using the microstrip lines shown in FIG. 15, however, the length of the pattern of the microstrip line is almost determined by the pass wavelength. Consequently, the reduction in the pattern occupying area is naturally limited and it is difficult to reduce the size of the device and substrate.
For example, as shown in FIGS. 16 and 17, what is called a triplate structure filter in which a conductor pattern is not formed in the surface layer of the substrate but a pair of conductor patterns are formed in an inner layer of a substrate having ground conductive layers on both sides has been proposed. FIG. 16 is a perspective view of the triplate structure filter. FIG. 17 is a plan view of the filter. As shown in the diagrams, the filter comprises a first substrate 111a made of a dielectric, a pair of conductor patterns 115(1) and 115(2) formed on the first substrate, and a second substrate 111b made of a dielectric staked on the first substrate 111a so as to sandwich the conductor patterns 115(1) and 115(2). A stacked substrate 111 comprised of the first and second substrates 111a and 111b is covered with a ground conductive layer 117 connected to the ground except for a pair of side end face areas 113(1) and 113(2).
The conductor pattern 115(1) functions as an input side conductor pattern and has a form in which a relatively wide conductor pattern 115(1)a as a low impedance line (hereinbelow, also referred to as a low impedance pattern for short) and a relatively narrow conductor pattern 115(1)b as a high impedance line (hereinbelow, also simply referred to as a high impedance pattern for short) are cascade connected. On the other hand, the conductor pattern 115(2) functions as an output side conductor pattern and has a form in which a relatively wide conductor pattern 115(2)a and a relatively narrow conductor pattern 115(2)b are cascade connected. The conductor patterns 115(1) and 115(2) are disposed at a predetermined interval so as to be in parallel to each other in the longitudinal direction. The narrow conductor patterns 115(1)b and 115(2)b are respectively connected in their intermediate parts in the longitudinal direction to an input part pattern 116(1) to which the RF signal RF1 is supplied and an output part pattern 116(2) from which the RF signal RF2 filtered in a band is outputted. One end of each of the narrow conductor patterns 115(1)b and 115(2)b is connected to the ground conductive layer 117 covering one end face of the stacked substrate 111.
As illustrated in FIG. 18, the filter is equivalently expressed in a form in which a parallel resonance circuit PR1 comprising a capacitor C1 and an inductance L1 connected between the input part pattern 116(1) and the ground and a parallel resonance circuit PR2 comprising a capacitor C2 and an inductance L2 connected between the output part pattern 116(2) and the ground are capacitive coupled to each other via a capacitor C3.
In the filter, the RF components except for the wavelength xcex of the RF signal RF1 supplied from the end of the input part pattern 116(1) are eliminated through the conductor patterns 115(1) and 115(2) functioning as the parallel resonance circuits PR1 and PR2. Only the RF signal RF2 of the wavelength xcex is outputted from the end of the output part pattern 116(2). According to the filter of the triplate structure, the area occupied by the conductor patterns can be reduced more than the microstrip filter shown in FIG. 15, so that the miniaturization of the BPF can be realized.
When the triplate structure filter is allowed to function as an equivalent circuit shown in FIG. 18, a filter of a comline type in which a pair of lines (conductor patterns) each having a length of the quarter of the pass wavelength xcex are capacitive coupled is usually employed. In the patterns shown in FIGS. 16 and 17, by cascade connecting the lines of different impedances, the line overall length La is made shorter than xcex/4, thereby realizing the miniaturization. In the following description, a BPF of such a type will be called a shortened comline type distributed constant BPF.
As described above, the BPF using the microstrip lines shown in FIG. 15 can be formed by one operation as a part of a pattern of the surface layer of a substrate in a wiring process of forming a wiring pattern on the surface layer of a wiring board by printing or lithography. For instance, as shown in FIG. 19, line connection between the BPF comprising the microstrip lines 102(1) to 102(5) and circuit chips such as an MMIC (Microwave Monolithic IC) 124 and chip capacitors 123(1) to 123(4) can be performed on a surface of the substrate 101. FIG. 19 is a plan view showing the configuration of a substrate module having the BPF using microstrip lines on the surface. Patterns 120(1) to 120(4) are ground conductive patterns, patterns 121(1) and 121(2) are power supply pads, and patterns 122(1) and 122(2) are power supply lines.
When the triplate structure BPF shown in FIGS. 16 and 17 is used in order to reduce the size of the substrate, however, the pair of conductor patterns 115(1) and 115(2), the input part pattern 116(1), and the output part pattern 116(2) are formed in the inner layer of the substrate. For example, as shown in FIG. 20, a conductor pattern area 137 in the BPF in the inner layer of the substrate 101 and connection pads 135(1) and 135(2) of the wiring pattern in the surface layer have to be connected to each other by using vias 136(1) and 136(2), respectively. More specifically, the input part pattern 116(1) (FIGS. 16 and 17) and the connection pad 135(1) have to be connected to each other by the via 136(1) and the output part pattern 116(2) (FIGS. 16 and 17) and the connection pad 135(2) have to be connected to each other by the via 136(2).
FIG. 20 is a plan view showing the configuration of a substrate module constructed by using the triplate structure BPF. The area 137 shown by an alternate long and short dash line corresponds to an area in which the conductor patterns in the inner layer are formed (that is, the area in which the pair of conductor patterns 115(1) and 115(2), the input part pattern 116(1) and the output part pattern 116(2) are formed). In the diagram, patterns 130(1) to 130(3) are ground conductive patterns, patterns 131(1) and 131(2) are power supply pads, patterns 132(1) and 132(2) are power supply lines, and patterns 138(1) to 138(4) are signal wiring patterns. The power supply wiring and the signal wiring patterns are connected to circuit chips such as MMIC 134 and chip capacitors 133(1) to 133(4) mounted on the surface of the substrate.
When the patterns in the inner layer of the substrate and the wiring patterns in the surface layer are connected to each other by the vias, parasitic inductance components (high impedance) of the vias are applied to the input/output parts of the BPF and it causes a change in the desired filter characteristics such as the center frequency and insertion loss.
As described above, the conductor patterns in the inner layer in the shortened comline type triplate structure BPF shown in FIGS. 16 and 17 has a form in which the low impedance wide pattern 115(1)a and the high impedance narrow pattern 115(1)b are cascade connected in order to reduce the size. There is a case such that the difference between the pattern widths is about ten times or more. Consequently, there is a fear such that the junction part between the low and high impedance patterns is subjected to a great stress by repetition of such as the temperature change and the performance of the filter deteriorates.
The invention has been achieved in consideration of the problems. The object of the invention is to provide a distributed constant filter, a method of manufacturing the distributed constant filter, and a distributed constant filter circuit module, capable of being connected to another wiring pattern or the like while maintaining the small size and eliminating the problems.
According to the invention, there is provided a distributed constant filter comprising: a substrate made of a dielectric; an input side conductor pattern which is formed on the surface or inside of the substrate and to which an electromagnetic signal is supplied; and an output side conductor pattern which is formed on the surface or inside of the substrate so as to sandwich the dielectric with the input side conductor pattern and outputs an electromagnetic signal in a frequency band as a part of a frequency band of the electromagnetic signal supplied to the input side conductor pattern, wherein at least one of at least a part of the input side conductor pattern and at least a part of the output side conductor pattern is formed to extend in the thickness direction of the substrate.
According to the invention, there is provided a method of manufacturing a distributed constant filter, comprising: a step of forming an input side conductor pattern and an output side conductor pattern on the surface or inside of a substrate made of a dielectric so as to interpose the dielectric between the patterns, the input side conductor pattern being supplied with an electromagnetic signal, the output side conductor pattern outputting an electromagnetic signal in a frequency band as a part of a frequency band of the electromagnetic signal supplied to the input side conductor pattern, wherein the step of forming the input side conductor pattern and the output side conductor pattern includes at least of: a step of forming a at least a part of the input side conductor pattern so as to extend in the thickness direction; and a step of forming at least a part of the output side conductor pattern so as to extend in the thickness direction.
According to the invention, there is provided a distributed constant filter circuit module comprising: a substrate made of a dielectric; an input side conductor pattern which is formed on the surface or inside of the substrate and to which an electromagnetic signal is supplied; an output side conductor pattern which is formed on the surface or inside of the substrate so as to sandwich the dielectric with the input side conductor pattern and outputs an electromagnetic signal in a frequency band as a part of a frequency band of the electromagnetic signal supplied to the input side conductor pattern; and a circuit chip disposed on the surface of the substrate and connected to the input side conductor pattern or the output side conductor pattern, wherein at least one of at least a part of the input side conductor pattern and at least a part of the output side conductor pattern is formed so as to extend in the thickness direction of the substrate.
In the distributed constant filter of the invention, at least one of at least a part of the input side conductor pattern and at least a part of the output side conductor pattern is formed so as to extend in the thickness direction of the substrate. An electromagnetic signal is supplied to the input side conductor pattern and an electromagnetic signal in a frequency band as a part of a frequency band of the electromagnetic signal supplied to the input side conductor pattern is outputted from the output side conductor pattern formed so as to sandwich the dielectric with the input side conductor pattern.
In the distributed constant filter of the invention, at least one of the input side conductor pattern and the output side conductor pattern includes a first conductor part and a second conductor part having different impedances. In this case, it is preferable that the part formed so as to extend in the thickness direction of the substrate is either the first or second conductor part having a higher impedance. Further, in this case, the conductor part having a higher impedance serves as an interlayer connecting part for connecting one of the plurality of conductor layers to another layer of those. In this case, the following configuration is also possible. Among the plurality of conductor layers, the conductor layer formed on the surface of the substrate functions as a wiring pattern to which a circuit chip is connected and the conductor layer formed inside of the substrate functions as either the first or second conductor part having a lower impedance.
In the method of manufacturing the distributed constant filter of the invention, in the step of forming the input side conductor pattern and the output side conductor pattern, the conductor part which extends in the thickness direction of the substrate and serves as at least a part of the input side conductor pattern is formed and the conductor part which extends in the thickness direction of the substrate and serves as at least a part of the output side conductor pattern is formed.
In the method of manufacturing the distributed constant filter of the invention, the step of forming the input side conductor pattern and the output side conductor pattern includes: a step of selectively forming a pair of conductor patterns functioning as a part of the input side conductor pattern and a part of the output side conductor pattern at an interval on a surface of a first dielectric substrate, the surface being opposite to the other surface on which a first ground conductor pattern is formed; a step of stacking a second dielectric substrate on the surface of the first dielectric substrate and combining the substrates to thereby form a single combined substrate; a step of selectively forming a pair of wiring patterns made of a conductor at an interval on the surface of the second dielectric substrate in the combined substrate; a step of forming a pair of through holes in the combined substrate so that the through holes allow each of the pair of conductor patterns to communicate each of the pair of wiring patterns, respectively; and a step of forming a pair of conductor functioning as another part of the input side conductor pattern and another part of the output side conductor pattern in the pair of through holes, to thereby make a electrical connection between each of the pair of conductor patterns and each of the pair of wiring patterns.
In the method of manufacturing the distributed constant filter of the invention, the step of forming the input side conductor pattern and the output side conductor pattern may comprise: a step of forming a pair of first through holes in a first dielectric substrate; a step of selectively forming a pair of conductor patterns functioning as a part of the input side conductor pattern and a part of the output side conductor pattern on one of the surfaces of the first dielectric substrate and forming a pair of conductors functioning as another part of the input side conductor pattern and another part of the output side conductor pattern in the pair of first through holes; a step of stacking a second dielectric substrate having a pair of second through holes formed in correspondence with the pair of first through holes of the first dielectric substrate on the surface on which the pair of conductor patterns are formed of the first dielectric substrate and combining both of the substrates to thereby form a single combined substrate; and a step of selectively forming a pair of wiring patterns made of a conductor at an interval on the surface of the second dielectric substrate in the combined substrate and forming another pair of conductors functioning as another part of the input side conductor pattern and another part of the output side conductor pattern in the pair of second through holes of the second dielectric substrate to thereby make electrical connections between each of the pair of conductor patterns formed on the surface of the first dielectric substrate and each of the pair of wiring patterns.
Other and further objects, features and advantages of the invention will appear more fully from the following description.